text: The free fraction is a parameter in pharmacokinetics and receptor-ligand kinetics.
  One speaks of two different free fractions:
  * Plasma free fraction, previously referred to as ƒ, is now referred to as ƒ according to consensus nomenclature.
  * Tissue free fraction (ƒ), previously referred to as ƒ
  The plasma free fraction is the fraction of the ligand at equilibrium in blood plasma that is not bound to plasma proteins.